1. Field of the Invention
The present invention relates to centrifugal filters for filtering particulates from a liquid using centrifugal force.
2. Description of the Related Art
Many types of fluids contain particulates which need to be filtered out for subsequent use of the fluid. Examples of such fluids include medical and biological fluids, machining and cutting fluids, and lubricating oils. With particular reference to an internal combustion engine, a lubricating oil such as engine oil may contain particulates which are filtered out to prevent mechanical or corrosive wear of the engine.
Diesel engine mechanical wear, especially that relating to boundary lubricated wear, is a direct function of the amount of particulates in the lubricating oil. A particulate which is extremely detrimental to engine wear is soot, formed during the combustion process, and deposited into the crankcase through combustion gas blow-by and piston rings scraping of the cylinder walls. Soot is a carbonaceous polycyclic hydrocarbon which has extremely high surface area whereby it interacts chemically with adsorptive association with other lubricant species. Particle sizes of most diesel engine lubricant soot is between 100 Angstroms and 3 microns. Ranges of concentration are between 0 and 10 percent by weight depending on many factors. Because engine wear will dramatically increase with the soot level in the lubricating oil, engine manufacturers specify a certain engine drain oil interval to protect the engine from this type of mechanical wear. Current sieve type filters do not remove sufficient amounts of soot to provide soot related wear protection to the engine.
Centrifugal filters for lubricant filtration are generally known. Current production centrifugal lubricant oil filters are powered by hero turbines, which are part of the oil filter canister, or through direct mechanical propulsion. Hero turbine powered filters are limited by the supplied oil pressure from the engine, and only can operate up to maximum speeds around 4000 revolutions per minute (RPM) with oil pressures nominally at less than 40 psi. In addition, hero turbine powered filters pass oil through the filter canister as it migrates toward the attached hero turbine jets. Therefore, the lubricant mean residence time is less than a few minutes. None of the currently available centrifugal filters which operate on the basis of a hero turbine provide satisfactory soot removal rates. Soot removal from engine lubricating oil requires greater G forces and longer residence times than is demonstrated with currently commercially available hero turbine powered filters.
It is also known to drive a centrifugal filter using a mechanical linkage from a turbine. The turbine receives a flow of engine exhaust air and drives a mechanical output shaft which in turn is coupled with a filter inside a centrifugal filter assembly. The rotational speed of the filter is sufficient to separate particulates within the engine oil. An example of such a filter is disclosed in U.S. Pat. No. 5,779,618 (Onodera, et al.).
All of the units described above and others commercially available fall generally in groups of hero turbine design or direct mechanical actuation. While direct mechanically driven systems are capable of reaching the necessary G forces to provide soot removal, this type of linkage is generally very expensive and requires extensive modification of engines to adapt. While hero turbines do not suffer from this problem, insufficient G forces limit these filters from removing soot.